1. Field of the Invention
The invention relates to a heat radiating structure of a printed circuit board and a printed circuit board producing method and is particularly designed to improve the heat radiating capability of a heat generating electronic part, such as an IC chip, to which a large current is applied and to prevent an occurrence of problems at the time of producing a printed circuit board having the heat radiating capability.
2. Description of the Related Art
A printed circuit board has a mounting surface for receiving and electronic part. The mounting surface has a land for soldering the electronic part. A through hole is formed in the land and heat is radiated to the other surface through the through hole.
For example, Japanese Unexamined Patent Publication No. H09-148691 and FIGS. 8(A), 8(B) and 8(C) herein disclose a heat generating element 1 mounted on a double-sided circuit board 2. More particularly, the circuit board 2 has opposite first and second surfaces 2a and 2b. A first copper foil flat pattern 3 is provided the first surface 2a of the circuit board 2 and the underside of the heat generating element 1 is secured to the first copper foil flat pattern 3 using solder or paste. The first copper foil flat pattern 3 is connected electrically to a second copper foil flat pattern 5 on the second surface 2b through viaholes 4. Conductive layers 4a are formed on the inner circumferential surfaces of the viaholes 4 by plating. Thus, heat generated by the heat generating element 1 is radiated from the second copper foil flat pattern 5 on the second surface 2b via the first copper foil flat pattern 3 and viaholes 4 is transferred efficiently from the first copper foil flat pattern 3 through the viaholes 4 and is radiated from the second copper foil flat pattern 5 on the second surface 2b. However, Japanese Unexamined Patent Publication No. H09-148691 simply discloses that the heat generating element 1 is secured to the copper foil flat pattern 3 using paste, solder or the like, and a specific securing method using solder or paste is unclear.
Further, Japanese Unexamined Patent Publication No. 2004-127992 discloses a printed circuit board with through holes that penetrate from the top surface to the under surface. The under surface is placed on a base plate. A resin paste is printed from the topside by a squeegee so that the resin paste fills the through holes and prevents solder flowing into the through holes from forming protuberances at the underside.
The method of using a paste or solder for securing the heat generating element 1 and the copper foil flat pattern 3 to a printed circuit board having the conductor patterns on both surfaces is unclear in Japanese Unexamined Patent Publication No. H09-148691. FIGS. 9A to 9D show how molten solder might flow into the viaholes 4 while securing the copper foil flat pattern 3 to the heat generating element 1 provided with a heat sink for heat radiation and electrical connection on the underside thereof. In such a case, the molten solder might be solidified at the underside and so-called solder balls 7a might project at the underside.
Specifically, cream solder 7 is applied to lands 6 that are solder-connected to lead terminals 1a of a heat generating element 1 on one surface 2a of a circuit board 2 and a copper foil flat pattern 3 for heat radiation as shown in FIG. 9A; a first reflow process is carried out by heating the circuit board 2 with the heat generating element 1 placed on the upper surface thereof as shown in FIG. 9(B); and the lead terminals 1a of the heat generating element 1 and the underside of the heat generating element 1 are soldered respectively to the lands 6 and the copper foil flat pattern 3.
The solder applied to the copper foil flat pattern 3 melts and can flow into the viaholes 4. Thus, the solder may spill from the openings surrounded by a copper foil flat pattern 5 on the second surface 2b, and solidifies as the solder balls 7a shown in FIG. 9C.
In this state, a second reflow process is carried out after cream solder is applied to the lands on the second surface 2b so that the copper foil pattern on the second surface 2b is soldered to terminals and an electronic part.
At this time, a metal mask 9 is mounted and the cream solder is applied by a squeegee 10 as shown in FIG. 9(D). However, the solder balls 7a cause the metal mask 9 to become uneven. Thus, the metal mask 9 and squeegee 10 may be damaged while applying the cream solder.
The heat of the reflow process separates the cream solder into solder and flux for facilitating the soldering. The flux flows into the viaholes 4 together with the molten solder to adhere to the copper foil flat pattern 5 on the underside. The flux is adhesive, and hence the metal mask becomes difficult to remove after the cream solder is applied with the metal mask mounted on the second surface, which presents a problem of reducing operability.
The through holes may be filled beforehand to prevent the formation of the solder balls 7a to close the through holes, as disclosed in Japanese Unexamined Patent Publication No. 2004-127992. However, this leads to an increased production cost because a resin applying step needs to be added.
The present invention was developed in view of the above problems, and an object thereof is to prevent improve the production process of a printed circuit board.